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HomeMy WebLinkAboutSWA000023_Calcs_20180427 EROSION CONTROL & STORMWATER MANAGEMENT DESIGN For North Buncombe Middle School Driveway & Site Improvements Prepared by: Mattern & Craig, Inc. 12 Broad Street Asheville, NC 28801 M. Scott Homewood, P.E. Originally Submitted March 6, 2018 Revised April 27, 2018 4/27/2018 - 2 - Table of Contents 1. Vicinity Map 2. Erosion Control Narrative 3. Stormwater Narrative and Calculations Summary 4. Copy of GIS Property Information (deed unavailable on-line) 5. Erosion and Sediment Control Calculations 6. Stormwater Management Calculations 7. Drainage Area Maps 8. 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The middle school in located north of Weaverville in Buncombe County. The driveway is planned primarily to reduce traffic congestion associated with dropping off and picking up students. In addition to the proposed construction, modifications will be made to the existing pavement markings to aid in congestion management. The project will entail disturbing approximately 3.35 Acres. The project site is a 28.95 Acre parcel located along North Buncombe School Road in unincorporated Buncombe County. The property is owned by the Buncombe County Board of Education. Site Description: The project site is located at 51 North Buncombe School Road and is bounded by North Buncombe School Road to the west and primarily single family residential homes on all other sides. The school buildings are positions on the high point of the site with moderate slopes down to the east and west of the parcel. There are several existing school structures and auxiliary buildings on-site as well as tennis courts and ball fields. Most of the parcel area will be left undisturbed with the work focused on the existing grass slope and wooded area west of the school. Adjacent Property: The property uses bordering this parcel are currently a mixture of wooded, residential and commercial. The vast majority of the parcel is surrounded by single family residential homes. Weaverville City limits border the parcel to the south and residentially zoned unincorporated Buncombe county R-3 border the site to the east and north. Soils: The site consists of the following soils (soils were taken from the USDA Online GIS soil map): Clifton Clay Loam (clay loam) - Hydrology Group B, K value of 0.57 to 1.98 in/hr Udorthents, loamy (sandy clay loam) - Hydrology Group C, K value of 0.06 to 19.98 in/hr Udorthents-Urban land complex(2 to 50 percent) - Hydrology Group A, K value of 0.06 to 19.98 in/hr Urban Land - Hydrology Group NA, K value of NA Those soils composed of the Clifton Clay Loam have a higher K value and may pose an increased risk of erodibility. The majority of the disturbed area is comprised of teh Clifton Clay Loam. However given the erosion controls shown on the plans and the requirement that the contractor stabilize cleared areas within 7 days this should minimize in erosion problems. The other soils on the site have lower K values which indicate that they should be more resistant to erosion. The contractor shall install and maintain all Erosion and Sediment Control measures shown on the plans which will help reduce and prevent sediment from leaving the project site. Erosion Control Measures: All Erosion and Sediment Control Measure shall comply with the latest edition of the North Carolina Department of Environment and Natural Resources Practice Standards and Specifications (NCDENR). - 5 - Construction Entrance 6.06: The construction entrance provides a buffer for vehicular traffic between the construction site and public roadways. The construction entrance aids in removing mud and sediment from construction vehicles prior to them entering public roadways, as well as controlling surface runoff and limiting dust. The construction entrance is a graveled area or pad where vehicles enter or leave the construction site. The construction entrance shall be constructed where shown on the plans and in accordance with the latest NCDENR Practice Standards and Specifications. Permanent Seeding 6.11: Permanent seeding is used on construction sites as the final stabilization for areas of exposed soil. Permanent seeding helps control runoff and erosion of the soils on the site by providing a perennial vegetative cover through seeding. Permanent seeding shall be applied to the areas shown on the plans and shall be in accordance with the details shown on the plans and the latest NCDENR Practice Standards and Specifications. Outlet Stabilization Structure 6.41: Outlet Stabilization will be used to control erosion and the outfalls for the Sediment/SWM Basins, and for the outfalls for other storm drain pipes and ditches on the project site. These structures prevent erosion by reducing the velocity of the flow leaving the pipe or channel and dissipating energy. Outlet Stabilization shall be constructed where shown on the plans and details and in accordance with the latest NCDENR Practice Standards and Specifications. Sediment Fence (Silt Fence) 6.62: The purpose of the silt fence is to retain sediment from small disturbed areas by reducing the velocity of sheet flows which in turn allows sediment to settle out. Silt fence consists of fabric buried at the bottom, stretched and supported by posts. The silt fence shall be constructed where shown on the plans and in accordance with latest NCDENR Practice Standards and Specifications. Check Dam 6.83: A check dam is a small temporary rock/stone dam constructed across a drainage ditch or diversion. These help reduce erosion in the channel by reducing the velocity of the flow in said channel. Rock check dams shall be constructed where shown on the plans and in accordance with the latest NCDENR Practice Standards and Specifications. Construction Sequence: 1. Obtain all required permits from NCDEQ for land disturbing activities. Attend pre- construction meeting with State/School staff. 2. Construct a stabilized construction entrance as shown on plans prior to performing any grading operations on site. 3. After installation of the construction entrance, and before commencing grading operations, install all perimeter Erosion Controls, including silt fence and diversion ditches shown on the erosion control plan. 4. Once perimeter controls have been established, construct outlet protections, drainage ditches and check dams as shown on the plans. Install additional Erosion Control traps and barriers as requested by School staff or the engineer. Permanent ditches are to be stabilized within 7 days of construction. 5. Once all interior site erosion control measures have been installed, contractor can commence with the clearing of the site in preparation for roadway grading. 6. Construct the cut/fill slopes shown on the plans for the driveway; stabilize slopes within 7 days if slopes are great than 3:1, slopes flatter than 4:1, stabilize within 14 days. 7. Install storm drain system including outlet protection. - 6 - 8. Stabilize all areas where existing pavement is being replaced with grass within 14 days of demolition being completed. 9. For areas that will be let idle for longer than 15 calendar days, apply temporary seeding or matting to help stabilize them until construction resumes. 10. Upon completion of construction, perform final stabilization of the site and remove all temporary sediment control measures. 11. Request final approval from NCDEQ. Maintenance: 1. Contractor shall inspect Erosion and Sediment control measures daily and after any rainfall. Any repairs or deficiencies shall be corrected immediately. A daily record log shall be maintained on the site by the general contractor and will record any deficiencies and the corrective measures taken. 2. Contractor shall monitor silt fences for sediment accumulation and remove the sediment deposits as necessary to provide adequate storage volume for the next storm. 3. All areas with less than 4:1 slope shall be stabilized within 14 days of final grade being reached. Slopes greater than 3:1 and ditches shall be stabilized within 7 days of final grade being reached. 4. Installation of additional erosion and sediment control measures may be necessary if requested by on-site inspection of NCDEQ staff or the owner's representative or the Engineer. Reference the North Carolina Erosion and Sediment Control Manual Section 6.01-6.02 for additional information. 5. Contractor shall perform a self-inspection of the Erosion & Sediment Control measures installed on this project as outlined in the Sediment Pollution Control Act at the end of each phase of construction to ensure continued compliance with the approved Erosion and Sediment Control Plans. The Contractor shall document the results of this inspection and any corrective action taken to bring the site into compliance with the approved Erosion and Sediment Control Plans for this project on either an inspection form or on the approved Erosion & Sediment Control Plans stored at the project site and shall make these available to review upon request. This inspection is separate and independent of the inspections required by the NPDES Stormwater Permit for Construction Activities. 6. Contractor shall follow the requirements spelled out in the NPDES Stormwater Permit for Construction Activities issued for this site. - 7 - STORMWATER MANAGEMENT NARRATIVE AND CALCULATIONS A stormwater collection and treatment system has been designed to collect runoff from the additional impervious area and to detain the post-construction runoff to pre-construction levels. The proposed system consists of sheet flow drainage from the proposed roadway and parking areas to swales and one HDPE pipe under the driveway, leading to a bioretention basin with an outlet structure and pipe. The selected design storm for the system is the 1-year, 24-hour storm as stipulated by the Buncombe County Stormwater Management Ordinance. The school site will be maintained as a single parcel and is not subject to the sale of lots. Future expansion may utilize currently unimproved portions of the parcel but no specific expansions are under consideration at this time. For purposes of this stormwater management design, the pre-construction condition shall be considered as including the facilities present on the site. The post-construction condition includes the proposed new drive, parking area, asphalt loading strip, as well as minimal new concrete sidewalks and related impervious surfaces. The portion of the parcel impacted by these construction activities and the associated pervious and impervious drainage areas are illustrated in the following exhibit – Drainage Areas N. Buncombe Middle School Renovations and Additions. A portion of the proposed driveway falls within the NCDOT right of way (ROW) for N Buncombe School Road. As the stormwater bioretention system cannot be located within this NCDOT ROW, the bioretention basin has been located just upslope of the ROW. The flow from the area above the ROW is over-detained to compensate for the minor flow from impervious area within the ROW. The portion of the parcel impacted by the proposed driveway project will drain into two existing catch basins along North Buncombe School Road. Ditch 1 in the Stormwater Management Plan redirects additional drainage areas from existing and proposed impervious areas to the new bioretention basin to offset additional stormwater runoff from the new impervious areas on-site. Drainage Area 1 drains to an existing roadway ditch along N Buncombe School Road. A small portion of the new impervious area drains to Drainage Area 2 which drains to a catch basin at the intersection of N Buncombe School Road and Old N Buncombe School Road. These flows then join together on the west side of N Buncombe School Road. An additional self-contained drainage area, Drainage Area 3, is shown to represent the inlet and pipe network that serves the tennis courts and directs the flow from this area to the same catch basin that Drainage Area 2 flow into. Drainage Area 3 is show in all calculations, no changes are proposed to this area and no changes to the impervious area or resulting flow occur. The following table summarizes the drainage areas considered for this project. - 8 - Drainage Area Pre- Construction Pervious Area (acres) Pre- Construction Impervious Area (acres) Post - Construction Pervious Area (acres) Post- Construction Impervious Area (acres) DA #1 1.84 0.73 1.94 1.73 DA #2 2.56 1.22 1.70 0.94 DA #3 0.07 0.45 0.07 0.45 DA #4 0.04 0.00 0.00 0.04 DA #5 0.05 0.00 0.05 0.00 The proposed stormwater management system captures the runoff from the proposed additional parking area as well as portions of the currently untreated existing impervious areas and the majority of the proposed new driveway where the topography and existing ROW allow. As noted above, a portion of DA#2 is over-detained to offset un-detained areas such that the net flow from the site is less than the pre-construction discharge. DA#4, which is a small sub-area of DA#1 will flow across the super elevated roadway to the catch basin across the street. There is also an 8’ wide asphalt strip planned for access to parked busses in the bus loading area which is designated as DA#5 in the drainage area maps. The area of approximately 2,200 sq ft of new built upon area is currently grass but will be replaced by asphalt that will sheet flow to the adjacent grass disconnected pervious area. As shown on the drawings, the proposed bioretention area is positioned just above the NCDOT ROW in the horseshoe bend of the new driveway. The bioretention areas will receive sheet flow from portion of the driveway and will receive flow from the parking area and remainder of the driveway via a vegetated ditch terminating in a culvert under the driveway directing flow to the forbay inlet end of the basin. The basin will provide 1-inch first flush retention and detention of the 24 hour, 1-year storm such that post-construction discharges are less than or equal to pre- construction discharges. Infiltration through the bottom of the basins, while likely to contribute slightly to the overall treatment capacity, is not considered and the basins are designed to treat the needed volume within the media and discharge is via the underdrains. Volumes in excess of the media storage capacity will pond on top of the media with a ponding depth of only 6” before exiting the grate in the outlet control structure. The discharge rate of the system is controlled primarily via the two-4” underdrain orifices. The 5-year, 24 hour storm event will generate flows just large enough to begin existing via the proposed outlet grate. All storms less than the 5-year storm will be released through the media and underdrain system. The swales and pipes are designed to convey the 25-year storm event. The outlet structure passes the 1000-year storm without utilizing the emergency spillway. HydroCAD software was utilized to model the performance of the system for the 24-hour, 1-year storm event and the 1-inch first flush event. The following HydroCad output demonstrates that the system will detain the design flow to pre-development discharge rates for the 1-yr 24-hr storm. The peak pre-development design-storm discharge is 7.54 cfs (HydroCAD Link 12Lof - 9 - the following calculations). The post development discharge from the site is reduced to 6.89 cfs HydroCAD Link 8L, page of the following calculations). Also included in the HydroCad stormwater modeling printouts are various calculations for all of the stormwater management components utilized on this project. The stormwater modeling includes calculations for the existing and proposed drainage area, ditches, proposed drain pipes and culverts, as well as the bioretention basin. GIS PROPERTY INFORMATION Land Data Total Acres: 28.95 Acreage Land Value: 393,100 Other Improvements Value: 0 COUNTY OF BUNCOMBE, NORTH CAROLINA Web Property Record Card 9743-44-1042-00000 << Back to Parcel Details Date Printed: 3/1/2018 Owner Information Owners: BUNCOMBE COUNTY BOARD OF EDUCATION Address: 175 BINGHAM RD ASHEVILLE NC 28806-3800 Property Location:51 N BUNCOMBE SCHOOL RD Taxing Districts County: Buncombe County City: Fire: NORTH BUNCOMBE School: Parcel Information Total Property Value: 393,100 Status: Active Deed Date: 4/5/1952 Deed Book/Page:0717 / 0626 Plat Book/Page: 0000 / 0000 Legal Reference: TRANSFER BY DEED Location:51 N BUNCOMBE SCHOOL RD Class: GVMNT/HIGH SCH. Neighborhood: DULA SPRINGS ROAD Subdivision: Sub Lot: Conservation/Easement: N Flood: N Ownership History No Owner History Assessment History Year Acres Land Bldgs Other Impr Assessed Desc Exemptions Deferred Taxable 2018 28.95 393,100 0 0 393,100 EXM 393,100 0 0 2017 28.95 393,100 0 0 393,100 EXM 393,100 0 0 2016 28.95 286,300 0 0 286,300 EXM 286,300 0 0 2015 28.95 286,300 0 0 286,300 EXM 286,300 0 0 2014 28.95 286,300 0 0 286,300 EXM 286,300 0 0 2013 28.95 286,300 0 0 286,300 EXM 286,300 0 0 2012 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2011 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2010 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2009 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2008 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2007 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2006 28.95 425,300 0 0 425,300 EXM 425,300 0 0 2005 28.95 564,500 0 0 564,500 EXM 564,500 0 0 2004 28.95 564,500 0 0 564,500 EXM 564,500 0 0 2003 28.95 564,500 0 0 564,500 EXM 564,500 0 0 2002 28.95 564,500 0 0 564,500 EXM 564,500 0 0 2001 28.95 86,000 0 0 86,000 EXM 86,000 0 0 Page 1 of 2Buncombe County Tax Lookup - Property Card 3/1/2018 Segment#Units Description 1 24.32 Acreage CLASS 1 IN AC 2 4.63 Acreage CLASS 2 IN AC Total Building Value: 0 Page 2 of 2Buncombe County Tax Lookup - Property Card 3/1/2018 EROSIONS AND SEDIMENT CONTROL CALCULATIONS